netdev - Re: [PATCH] dsa: tag_rtl4_a: Bump min packet size

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Date: Sun, 29 Oct 2023 16:38:04 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Florian Fainelli <f.fainelli@...il.com>
Cc: Andrew Lunn <andrew@...n.ch>, Vladimir Oltean <olteanv@...il.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dsa: tag_rtl4_a: Bump min packet size

On Fri, Oct 27, 2023 at 11:23 PM Florian Fainelli <f.fainelli@...il.com> wrote:

> > It turns out that sometimes, but not always, small packets are
> > dropped by the switch for no reason.
>
> And we are positive that the Ethernet MAC is also properly padding
> frames before having them ingress the switch?

I don't fully follow, this code is the one adding the padding isn't it?
Then the result is transmitted to the switch from the ethernet
MAC (drivers/net/ethernet/cortina/gemini.c).

What am I getting wrong here...

> > If we pad the ethernet frames to a minimum of ETH_FRAME_LEN + FCS
> > (1518 bytes) everything starts working fine.
>
> That is quite unprecedented, either the switch is very bogus or there is
> something else we do not fully understand...

The switch is pretty bogus, all documentation we have of it is a vendor
code drop, no data sheet. The format for ingress and egress tags
was discovered using trial-and-error.

Yours,
Linus Walleij